The invention relates to an imaging method including the following steps:
emitting at least two successive pulses;
receiving reflection or matter excitation signals, generated by the object body in response to said two pulses;
processing the received signals, in a manner dependent on their nature and transmission mode
combining together said two received signals and transforming the combined signal into image data related to the pulses emitted into the object body.
Imaging is widely used as a highly non-invasive diagnostic method. This technique substantially consists in generating electromagnetic or acoustic waves propagating in one direction and in observing electromagnetic radiation caused by relaxation of the matter previously excited by the electromagnetic waves transmitted to the object body or the echoes generated when the transmitted acoustic wave is reflected and impacted against the interfaces between regions having different densities. For a predetermined direction of the acoustic wave, the so-called line of view, or for a predetermined focus of the electromagnetic waves in a section plane of the object body, image pixels are generated as being positioned in a two-dimensional image plane or in a three-dimensional image volume and as having a brightness corresponding to the information contained in the received response pulses.
Two prior art basic techniques are used with the above method, i.e. ultrasound imaging and Nuclear Magnetic Resonance.
The ultrasound imaging method consists in generating an ultrasonic acoustic wave propagating in one direction (line of view). Reflection signals are received and image pixels are generated by generating a point having a brightness proportional to the echo amplitude at a predetermined coordinate which is defined as a function of time after the acoustic wave pulse transmitted by the object body in the direction of the desired line of view.
In Nuclear Magnetic Resonance imaging, electromagnetic pulses are excited from nuclei as a response to the emission of radio frequency electromagnetic pulses. In this case, instead of a single line of view, for each pulse transmitted to the object body, slices of the latter are selected and the matter relaxation echoes resulting from the excitation pulses are reprocessed as a function of the slice selected during emission, to be univocally correlated to single image points of the slice corresponding to the one selected during excitation.
Both imaging techniques have considerable drawbacks which, in certain cases, hinder a correct interpretation of images and correspondence with reality. Said problems are mainly caused by thermal noise and by the production of object body motion and micro-motion artifacts and, particularly in ultrasound imaging associated with the use of contrast agents, by an incomplete rejection of the highly reflective regions of the body which are not perfused by contrast agents. These incompletely rejected regions generate a “Clutter” artifact which can overlap, and possibly hide, the contrast agent signal.
Several prior art echo signal processing techniques are known which try to restrict or filter out noise and/or artifacts in ultrasound contrast imaging i.e. in the ultrasound imaging carried out with contrast agents injected in the object body. These techniques provide complex echo signal filtering and/or autocorrelation procedures aimed at the recognition of unrelated signal components (such as noise), and at the removal or limitation of these components. Prior art procedures complicate the structure of the apparatus implementing them and also require comparatively long processing times.
The latter shortcoming is in contrast with the needs to obtain visible images in the shortest time, i.e. in real time, these needs being almost compulsory in ultrasound imaging combined with contrast agents.
In Nuclear Magnetic Resonance imaging several techniques are also known for reducing thermal noise and filtering out artifacts, which are also considerably complex and require long processing times and/or a complication of the hardware structure of imaging systems.
Therefore, the invention has the object to provide a method as described hereinbefore, which allows to obviate the above drawbacks in a fast and inexpensive manner and without involving a more complex construction of the apparatus.
The invention achieves the above purposes by providing a method as described above, which includes the following steps:
combining the response signals, i.e. the echoes relating to the two successive ultrasonic or electromagnetic pulses in an electromagnetic pulse excitation sequence, by a weight function which, by comparing corresponding samples of the two response signals, assumes values in a range between a maximum value and a minimum value depending on the mutual correlation measure between said corresponding samples of the two response signals.
combining the weight function thereby obtained with the combination of the two response signals and transforming the resulting signal into image data, i.e. image points (pixels, voxels).
According to a further characteristic, the weight function has two predetermined maximum and minimum values which are assumed when the corresponding components of the two successive response signals have equal or opposite phases respectively, whereas in case of partly unrelated signal components the function assumes intermediate values.
It is possible to use a correlation weight function having a continuous development or a function having discrete values.
Advantageously, the method of the invention provides response signal sampling before processing.
Moreover, the method of the invention may provide filtering of the at least two successive response signals related to the two successive identical pulses before their combination with each other and/or with the weight function and/or after their combination with each other and/or with the weight function.
An advantageous correlation determination function to implement the method is the function EXNOR applied to the signs of the samples of the two successive response signals.
The combination of the two response signals related to the two identical successive ultrasonic or electromagnetic pulses transmitted to the object body may be obtained by any function, for instance a signal subtraction or addition or multiplication or division function.
The weight function may be further averaged, e.g. by integration or low-pass filtering.
The method of the invention may be also implemented with functions other than the EXNOR sign function, and providing values in the range between a minimum value and a maximum value depending on the occurrence of predetermined conditions of comparison between the at least two response signals to the two identical successive pulses transmitted to the object body.
According to a further variant, particularly relevant in ultrasound imaging but also applicable to Nuclear Magnetic Resonance imaging, the correlation weight function, possibly averaged, or integrated or filtered, may be combined with a thresholding function, which assigns the value 0 if the response signals exceed a predetermined threshold and the value 1 if they are below a predetermined threshold, or vice versa. The two threshold values may generally be different between the two vectors P1 and P2 representing the two received and sampled response signals. Two different threshold signals will provide a more progressive threshold application.
A function suitable for the purpose is the logic NOR function applied to the most significant N bits of the signal value. In fact, the signal is represented in a Kbit scale plus a sign bit. In ultrasound imaging, for instance, due to the high reflectivity differences between blood (or contrast agent) echoes and the reflecting CLUTTER generating structures, which differences are of the order of 1:100, a threshold level is easily obtained. The total Kbits may be divided into K1+N where K1+N=K. The signal shall be kept if it falls within the first K1 bits, and attenuated or zeroed if it reaches the bit K1+1. If different thresholds are provided, the most significant N bits for the P1 signal and the most significant M bits for the P2 signals will be considered.
The thresholds are determined with reference to the number (N, M) of most significant bits being considered in the NOR function. The two P1 and P2 signals may be also evaluated by considering their magnitudes.
Thresholds may be also determined by referring to the most significant N bits of one of the two vectors considered in its magnitude.
The techniques for focusing the emitted ultrasonic beams, for reconstructing the vectors related to the echo signals and for image processing/reconstruction from echo signals are conventionally known. In this case, both two- and three-dimensional scanning and reconstruction techniques may be used. The above also applies to the implementation of the method of the invention to Nuclear Magnetic Resonance imaging techniques.
The invention also pertains to an ultrasound imaging system for implementing the above method, which includes
at least one transducer for transforming electric signals into an ultrasonic pulse, preferably a geometrically and numerically predetermined transducer array;
at least one receiving transducer, the same as the transmitting transducer or separate therefrom, preferably a geometrically and numerically predetermined receiving transducer array, which may be the same as the transmitting transducer array or separate therefrom;
means for controlling the transmitting and receiving transducers for alternate transmission and reception activation;
means for focusing ultrasonic beams in a certain propagation direction, i.e. along a predetermined line of view by synchronized activation of the transmitting transducers, when a transmitting transducer array is provided;
means for focus reconstruction relative to the received echo signals, when a receiving transducer array is provided, by resettling synchronization relative to the signals received by the individual transducers, with reference to transmission synchronization;
means for sampling the received echo signals;
means for combining together two successive received echo signals;
means for processing the received echo signals to remove the undesired signal components;
means for transforming the processed echo signals into image signals related to at least one point or one line of a three- o two-dimensional image formed by a set of points (pixels or voxels) or by a set of lines.
With reference to this invention in its most general form, the above system provides that the means for processing the received echo signals which allow removal of undesired signal components comprise means for weighting the received signals based on the mutual correlation of identical or corresponding samples of two echo signals related to two successively emitted transmission pulses having equal or opposite phases.
A preferred correlation rule consists in comparing the relative phase conditions of the samples corresponding to the two echo signals, the weight being determined in a range between a maximum value and a minimum value depending on phase coincidence or phase opposition conditions. Phase (as is known from literature) may be determined by the EXNOR function applied to the sign of two corresponding samples of the two P1 and P2 vectors.
The method and system of the invention allow to obtain a simple noise reduction and to limit, i.e. reduce noise. Moreover, the weight function also allows, in the condition of use with opposite phase transmission signals, to remove or anyway limit the generation of artifacts or clutter, generated for instance in ultrasound second harmonic imaging with the Pulse Inversion method. In fact, in this case, the different processing methods used to date provide removal of the received echo signals caused by two successive pulses of opposing phases (Pulse Inversion). Such removal occurs based on the sum of the samples of the two received echo signals, and only leads to a partial rejection of the motion undesired signals (Clutter). The weight function in this case drastically decreases clutter (any fundamental frequency signals of opposing phase being emitted are interpreted as “unrelated” thereby increasing rejection) as well as the unrelated thermal noise between the two scans.
Therefore, the method and system of the invention, besides providing the inexpensive and fast possibility to remove or at least reduce noise, also implement the basic processing steps, or at least a few basic processing steps of some ultrasound second harmonic imaging techniques.
From the practical point of view, the method of the invention may be inexpensively implemented in an ultrasound imaging system or apparatus or in a Nuclear Magnetic Resonance system or apparatus and its application is fast.
The method, i.e. the steps in which the samples are compared, interpreted and weighted, may be easily implemented in suitable dedicated or programmable hardware. Those skilled in the art can both create a logic circuit operating according to the selected logic function to define the weight function, e.g. the EXNOR function, and to appropriately program any programmable hardware. The choice between these two opportunities also depends on the general structure of the system or apparatus with further reference to other features that are beyond the scope of this invention.
It shall be noted that the method of the invention, hence the system and apparatus for implementing it may be provided in combination with several imaging modes. Particularly, the method of the invention may be used, for instance, with the conventional imaging technique, in which the received echo signals are processed and evaluated with reference to the fundamental frequency component. In this case the method of the invention allows to reduce noise and to provide output image signals having an optimized signal to noise ratio.
Another application is in the field of Harmonic Imaging, either with or without contrast agents. In this imaging mode, echo signals are processed with reference to a harmonic component of the fundamental frequency, typically with reference to the second harmonic. Therefore, the fundamental component must be removed from echo signals and in this case, the method of the invention allows to remove in a simple, fast and inexpensive manner the effect of the generation of undesired motion artifacts or clutter, due to the fact that when tissues of parts of the body move, uncontrolled signal dephasing occurs, which gives non-zero contributions upon processing to remove the fundamental frequency component of the received echo signals.